Numerous systems exist in the art wherein electrically operable elements, e.g., heating elements, valves, pumps, blowers, and like, must be available for operation on a continuous basis to ensure satisfactory operation of the over-all system which would produce disastrous results in terms of damage to other equipment in the system and the like. Various techniques have been proposed to overcome or minimize the problems due to a failure of such a "key" operable element. Such prior techniques, however, have suffered from one or more drawbacks commercially in being unable to distinguish adequately the primary load's failure, unduly complex and expensive, requiring supplemental power sources, and/or not providing a satisfactory alarm of the failure of the primary element.
Accordingly, the art has continued to search for simple and inexpensive, reliable back-up and alarm systems for use with "key" electrically operable elements.